A phosphor emits a light by applying radiations such as UV light, electron beams, or X-rays, or by application of electric fields. Various phosphors have been hitherto studied and practically used for various purposes such as illumination (luminescent lamps), image displaying, and radiation measurement. Display devises comprising phosphors are also known, for instance, CRT and VFD. On the other hand, a switching element or device works for switching between the states of "on" and "off", and a delay element or device works for making a delay of transmission of signals.
U.S. Pat. No. 5,391,884 discloses a phosphor and an X-ray imaging plate comprising the phosphor. The disclosed phosphor contains an activated gadolinate host, and the host is essentially composed of the complex composition having the following formula: EQU BaGd.sub.2 O.sub.4 :yTb,zSm
wherein, y and z are numbers satisfying the conditions of O&lt;y&lt;1, O&lt;z&lt;5, and 1.times.10.sup.-5 &lt;y+z&lt;6.0). SUMMARY OF THE INVENTION
The present inventor made studies about rare earth activated divalent metal-trivalent metal oxide phosphors, and finally found the following facts.
When a terbium-samarium co-activated alkaline earth metal rare earth oxide phosphor is excited with a radiation (e.g., UV light X-ray), it emits luminescence comprising both green component (which is thought to originate from Tb.sup.3+) and red one (which is thought to originate from Sm.sup.3+). Each luminescent component has a different response time to the excitation, and the ratio between them depends on the alkaline earth metal and the rare earth (which are the center metal atoms in the phosphor matrix). Therefore, the time difference between these luminescent components can be controlled by selecting the center metal atoms in the phosphor matrix.
Accordingly, it is an object of the present invention to provide a phosphor suiting display material and showing dichromatic emission with a time difference.
It is another object of the invention to provide a phosphor appropriately employable as a switching or delay element or device and showing dichromatic emission with a time difference.
The invention resides in a method for displaying plural colors with time difference which comprises applying radiation to a terbium-samarium co-activated alkaline earth metal rare earth oxide phosphor which is composed of a composition of the formula (I): EQU M.sup.II M.sup.III.sub.2 O.sub.4 :xTb,ySm (I)
in which M.sup.II is at least one alkaline earth metal element selected from the group consisting of Mg, Ca, Sr and Ba; M.sup.III is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; and x and y are numbers satisfying the conditions of 0&lt;x.ltoreq.0.1 and 0&lt;y.ltoreq.0.1, respectively, to produce with time difference plural emissions of different wavelengths.
The invention also resides in a method for transmitting plural signals with time difference which comprises applying radiation to a terbium-samarium co-activated alkaline earth metal rare earth oxide phosphor which is composed of a composition of the formula (I): EQU M.sup.II M.sup.III.sub.2 O.sub.4 :xTb,ySm (I)
in which M.sup.11 is at least one alkaline earth metal element selected from the group consisting of Mg, Ca, Sr and Ba; M.sup.III is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; and x and y are numbers satisfying the conditions of 0&lt;y.ltoreq.0.1 and 0&lt;y.ltoreq.0.1, respectively, to produce with time difference plural emissions of different wavelengths and
collecting the plural emissions separately by plural photoelectric conversion elements which emit electric signals with time difference.
The invention furthermore resides in a terbium-samarium co-activated barium rare earth oxide phosphor comprising the composition represented by the formula (I"): EQU BaM.sup.III.sub.2 O.sub.4 :xTb,ySm (I")
in which M.sup.III is at least one rare earth element selected from the group consisting of Y, La and Lu; and x and y are numbers satisfying the conditions of 0&lt;x.ltoreq.0.1 and 0&lt;y.ltoreq.0.1, respectively.